1. Field of the Invention
The present invention relates to a battery monitoring system, and to a method of diagnosis for a battery monitoring system.
2. Description of Related Art
With a hybrid automobile or an electric automobile or the like, in order to ensure the desired high voltage, a group battery structure is employed that includes a large number of battery cells that serve as secondary batteries, connected in series. With this type of group battery, in order to provide capacity calculation and protection management for each of the battery cells, management of the battery cells is performed using monitor ICs that monitor the states of the battery cells and control ICs that control their charge/discharge states. In particular, since there is a danger of an excessive charge state occurring with a system that employs a lithium ion battery due to the high energy density of such a lithium ion battery, accordingly, as disclosed in Japanese Patent No. 4,092,580, the reliability and the security are enhanced by the voltage of each of the cells being measured by the control ICs and the monitor ICs so that any excessive charge state is detected, and by it being arranged to stop the charge or discharge of the battery if such an excessive charge state is detected by any of the ICs.
The monitor ICs detect the voltages of each of the battery cells individually; for example, if there is some battery cell that is in an excessive charge state, then excessive charge information is transmitted by communication with its corresponding control IC. And, in order to ensure that such excessive charge information will be reliably transmitted to the control ICs, by transmitting test signals from the control ICs, diagnosis is performed for determining whether or not there is any anomaly such as breakage of a communication line or the like.
Furthermore, when detecting the voltages of a battery cell, a predetermined battery cell is selected by a multiplexer, and its voltage is detected by a voltage detection unit. Thus, it is possible to detect the voltages of all of the battery cells by changing over the connections of the multiplexer. In order to acquire the correct cell voltages, it is necessary for a cell voltage measurement circuit including the multiplexer to operate correctly. Due to this, in Japanese Laid-Open Patent Publication 2008-92656, it has been proposed to compare together the sum of all of the individually measured values for the voltages of all of the cells, and the measured value for the total battery voltage as measured by a total voltage measurement circuit, and to decide that there is some fault with the cell voltage measurement circuit including the multiplexer, if there is a significant difference between these two.
However, in the above described excessive charge diagnosis process, it is not possible to proceed as far as diagnosing whether or not the excessive charge detection circuitry internal to the monitor ICs is functioning normally, and this is a deficiency from the point of view of enhancing the reliability. Furthermore, in the case of a system that decides upon whether or not there is a fault with the cell voltage measurement circuit by comparing together the sum of the individual voltages of all of the cells and the total voltage of them all together, if for example an anomalous condition occurs in which the multiplexer selects one cell only, but the states of charge of all of the cells are uniform, then there is almost no difference between the sum of the individual cell voltages and the total voltage, so that there is a fear that an erroneous diagnosis of normal operation will be reached. Moreover, if a sudden voltage fluctuation occurs simultaneously with the measurement of the voltage of one cell that is being selected by the multiplexer, then it may happen that this causes a difference between the sum of all of the individual cell voltages and the voltage of them all together, so that there is a risk of an erroneous decision even though the multiplexer is actually operating correctly. Accordingly, in order reliably to detect excess charging, as described above, it has been necessary to enhance the reliability of the monitor ICs and the control ICs separately.